The present invention relates to process variable transmitters used in process control and monitoring systems. More specifically, the present invention relates to process variable transmitters which sense a temperature of a industrial process fluid using a thermocouple.
Process control transmitters are used to measure process parameters in a process control or monitoring system. Typically, the transmitter includes some type of a process variable sensor having an output which is digitized by an analog to digital converter and provided to an microprocessor. One type of process variable sensor is a temperature sensor which is used to sense a temperature of a process fluid. The sensed temperature can be used directly, or can be used to compensate another process variable such as flow. The process variable is transmitted from the remote location to a local location over a process control loop. The process control loop can comprise, for example, a two wire process control loop or other configuration, including a wireless configuration.
One type of temperature sensor is a thermocouple which is formed when two different types of metals are placed into contact. A voltage is produced between these two metals which is related to the temperature of the junction. This voltage can be measured and, if desired, digitized by circuitry in the transmitter. The thermocouple has two wires which are configured to connect to first and second electrical connectors of the transmitter. However, in order to obtain accurate temperature measurements, the orientation (i.e., polarity) of the thermocouple with respect to the first and second electrical connectors must be known.